1. Field of the Invention
The invention relates to a method for compensating channel errors in digital data communication of a signal in sampled data form, such as errors, phase noise and frequency offset caused by multipath propagation in a digital data communication system, especially an OFDM system (Orthogonal Frequency Division Multiplexing).
2. Discussion of Related Prior Art
In this disclosure channel stands for a transmission path used in data transmission, which usually is a radiopath in wireless data transmission systems.
In wireless airborne data transmission, the signal will always be distorted to some extent in practice when passing from the transmitter to the receiver. Distortions are caused among other things by buildings, woods and meteorological variations. Moreover, as the receiver and/or the transmitter moves, the conditions will change constantly, thus affecting flawless signal reception. In practice, a signal reaches the receiver along a number of paths, and thus the distances covered by the signals following different paths will not be equally long, generating phase and amplitude differences between the signals. Imperfect oscillators also entail phase errors, for instance.
The OFDM system uses several subcarrier wave frequencies, allowing several signals to be transmitted at the same time. Each signal X.sub.m serves to modulate one subcarrier wave. The modulated subcarrier waves are combined and transformed into a mode suitable to be transmitted on the transmission path. In the receiver, demodulation is carried out, separating the transmitted signals to various receiving channels. One known OFDM modulator can be formed by using Inverse Discrete Fourier Transformation (IDFT), and accordingly, an OFDM demodulator can be formed with Discrete Fourier Transformation (DFT). FIG. 1 illustrates the principle of the OFDM system as a schematic block diagram. In FIG. 1 the channel block represents a transmission path, such as a radiopath, and then h(n) represents n transfer functions of the channel.
Prior art methods for compensating linear channel distortions are based on linear equalizers. In the OFDM system linear equalization is most frequently performed in the frequency domain owing to the nature of the OFDM system. Prior art methods are mainly based on minimizing the mean square error (MSE) of received and sent samples. To achieve this, adaptive algorithms are used, such as the least mean square (LMS) principle. Another option is to use a zero forcing (ZF) criterion in the frequency domain combined with a proportional algorithm (PA), which in practice provides faster convergence and also easier selection of the learning constant than adopting the MSE criterion in connection with an LMS algorithm.
Both the previously known methods mentioned above use the same type of method for compensating received and demultiplexed samples. The received samples are demultiplexed with Discrete Fourier Transformation (DFT), i.e. by transforming the received samples from time domain to the frequency domain, and subsequently the samples are multiplied with complex numbers, which are adjusted to the algorithm applied.
A known method that improves the compensation of constant phase errors uses minimum mean square error to estimate phase offset. This enables the frequency offset to be calculated and tuned to the oscillators.
The problem of LMS and PA algorithms is that when a small error is aimed at, the algorithms converge slowly. Then the algorithms are unable to follow fast variations in the channel characteristics. This causes problems especially regarding relatively long OFDM symbols, in which a phase change may be notable even during one single symbol duration.